1. Field of the Invention
The present invention relates to a battery feed circuit for a pair of subscriber lines, more particularly to a battery feed circuit with an internal termination, wherein the battery feed circuit itself forms an alternating current terminal, which is constituted to provide a desired alternating current terminal impedance.
2. Description of the Related Art
A subscriber line circuit in a digital switching system is provided with so-called BORSCHT functions, e.g., battery feed, overvoltage protection, ringing, supervision, codec, hybrid, and test functions. The present invention deals with a battery feed circuit which is arranged in a subscriber line circuit in order to carry out the battery feed function.
Battery feed circuits for supplying a speech current of direct current from an exchange to subscriber lines or trunk circuits are generally constructed to provide a direct-current feed resistance of, for instance, 200 ohms or 220 ohms and a high alternating-current terminal impedance to prevent a voice signal from transmission. In a subscriber line it is required that alternating current be terminated by a predetermined value (for instance, 600 ohms) of alternating current impedance. For satisfying these requirements, there is available in the prior art a battery feed circuit which uses inductance elements such as a transformer, coil or other devices and a termination circuit with a predetermined alternating current impedance. Such battery feed circuits, however, are not suitable for recent integrated circuits and large-scale integrated circuits. For this reason, it is expected that the battery feed circuit be constituted only by electronic components such as transistors. As it is necessary that such electronic feed circuits operate as inductance components, they are designated electronic inductance circuits. They have an alternating current component with an impedance of tens of kilohms or more. Since they exhibit a very high impedance in the alternating current component, a terminal circuit having a terminal impedance of, for instance, 600 ohms must be added. Here, it should be noted that while it is possible to cancel alternating current induced voltage having a longitudinal mode signal with coils or transformers, this is not possible with the above-described electronic inductance circuit. In other words, since the ground impedance against the induced noise is high, e.g., tens of kilohms, the alternating current induced voltage cannot be suppressed and it becomes excessively high, surpassing the power supply voltage of the internal components, such as operational amplifiers in the electronic inductance circuit. This results in saturation distortion in the alternating current signal and thus distortion in the voice signal. Therefore, there is the disadvantage of insufficient ability to withstand alternating current induced noise.